The effects of oxatomide, an anti-allergic drug, on the actions of chemical mediators were investigated in guinea pigs, rats and cats; and the following results were obtained: Studies on the guinea pig ileum revealed oxatomide to be a potent antagonist of histamine with a dual type of action, being competitive at low doses and noncompetitive at higher doses. Oxatomide at concentrations of 0.1 microM or higher inhibited the contractile responses evoked by crude SRS-A from sensitized guinea pig lung. Oxatomide (0.1 to 10 microM) did not inhibit the chronotropic effect of histamine in guinea pig atrium. In anaesthetized guinea pigs, serotonin-induced bronchoconstriction was antagonized by oxatomide (0.1 to 1 mg/kg, i.v.) as effectively as histamine-induced bronchoconstriction. However, oxatomide (up to 1 mg/kg, i.v.) did not inhibit acetylcholine-induced or arachidonic acid-induced bronchoconstriction. Oxatomide given orally in the range of 5 to 30 mg/kg markedly inhibited the increased vascular permeability by histamine, serotonin, and bradykinin in rats. Oxatomide at doses of 0.03 mg/kg (i.v.) or higher also prevented the contraction of the nictitating membrane induced by serotonin in cats. Oxatomide (0.3 microM and 1 microM) inhibited competitively the calcium-induced contracture of fully-depolarized taenia coli of guinea pigs. From these results, oxatomide appears to have potent antagonistic activities on the actions of various chemical mediators. These properties of oxatomide may contribute to its anti-allergic activity.